The Characteristics of the Fish and Crab Assemblages of the Leschenault Estuary. Inter-Period Comparisons and Their Management Implications
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The characteristics of the fish and crab assemblages of the Leschenault Estuary. Inter-period comparisons and their management implications. Lauren Veale, Peter Coulson, Steeg Hoeksema, James Tweedley, Norm Hall and Ian Potter Centre for Fish and Fisheries Research, Murdoch University South West Development Commission Report December 2010 The characteristics of the fish and crab assemblages of the Leschenault Estuary. Inter-period comparisons and their management implications. Lauren Veale, Peter Coulson, Steeg Hoeksema, James Tweedley, Norm Hall and Ian Potter* *Contact author Email: [email protected] Centre for Fish and Fisheries Research School of Biological Sciences and Biotechnology Murdoch University South Street, Murdoch Western Australia, 6150 Ph: 9239 8801 Fax: 9360 6303 This work is copyright. Except as permitted under the Copyright Act 1968 (Cth), no part of this publication may be reproduced by any process, electronic or otherwise, without the specific written permission of the copyright owners. Neither may information be stored electronically in any form whatsoever without such permission. 2 Table of Contents Executive Summary ...................................................................................................................... 4 Introduction ................................................................................................................................. 10 Materials and methods ............................................................................................................... 15 Sampling of fishes and crabs ......................................................................................... 15 Treatment of samples .................................................................................................... 17 Environmental variables ................................................................................................ 17 Assigning life cycle guilds to fishes .............................................................................. 18 Statistical analyses – fish community ........................................................................... 18 Statistical analyses – Blue Swimmer Crab assemblage ................................................ 22 Results .......................................................................................................................................... 23 Environmental variables ................................................................................................ 23 Abundances and numbers of fish species ...................................................................... 23 Densities of commercial and recreational fish species.................................................. 27 Numbers and percentage contributions of the different life cycle guilds ..................... 27 Compositions of the fish faunas .................................................................................... 29 Numbers of species and densities of fish ...................................................................... 39 Sex ratio and minimum legal carapace width for Blue Swimmer Crabs ...................... 42 Densities and catch rates of Blue Swimmer Crabs ....................................................... 42 Carapace width frequency distributions for Blue Swimmer Crabs ............................... 43 Historical comparisons of densities and catch rates of Blue Swimmer Crabs .............. 48 References .................................................................................................................................... 54 3 Executive Summary A total of 14,200 fishes was caught in the lower and middle regions of the Leschenault Estuary using a 21.5m seine net in each season between winter 2008 and autumn 2010. This total was only 3% less than the 14,601 fishes caught using the same seine net at the same sites twice seasonally in 1994, i.e. with the same amount of fishing effort. The numbers of species recorded in 2008-10 (36) and 1994 (33) were also similar. The above absence of a marked difference in the abundance of fish is consistent with the similarity in the mean densities of fishes per sample in the two periods. However, the mean number of species per sample and measures of diversity were greater in the current than earlier period. The eight most abundant species in 2008-10 ranked among the 11 most abundant species in 1994. Furthermore, the five most abundant species in 1994, which collectively accounted for ~ 90% of the total number of fish caught in that period, ranked amongst the top six species in 2008-10, recognising, however, that they contributed less, i.e. ~ 69%, to the total number of fish in that later period. These five species were the Elongate Hardyhead Atherinosoma elongata, the Sandy Sprat Hyperlophus vittatus, the Yelloweye Mullet Aldrichetta forsteri, the Silverfish Leptatherina presbyteroides and the Southern Longfin Goby Favonigobius lateralis. The far greater contributions of the most abundant species in 1994 than 2008-10 reflects the extreme dominance of the Southern Longfin Goby and Sandy Sprat in the earlier period. These two species thus contributed nearly 70% to the total number of fishes caught in the earlier period, compared with only 35% by the two most abundant species, i.e. Elongate Hardyhead and Sandy Sprat, in the later period. This helps account for the diversity of the fish fauna being less in the earlier period. 4 In contrast to the very high ranking of the Elongate Hardyhead, the Sandy Sprat, the Yelloweye Mullet, the Silverfish and the Southern Longfin Goby in both periods, the Spotted Hardyhead Craterocephalus mugiloides and the Common Hardyhead Atherinomorous vaigiensis ranked fifth and ninth, respectively, in terms of abundance in 2008-10, but were not caught in 1994. Furthermore, these two species of Hardyhead were likewise relatively abundant in additional samples taken in the Leschenault Estuary using a 41.5 m seine in 2008-10 and were never caught using that net in 1994. As these two Hardyheads are tropical species, and have not previously been recorded south of the Peel-Harvey Estuary, their current substantial numbers in the Leschenault Estuary represent a southwards extension of their distribution, presumably in response to the increases in coastal water temperatures that have occurred along the lower west coast during recent years (Pearce and Fang, 2007). The above inter-period differences help account for the species compositions in 1994 and 2008-10 being significantly different. However, the most important species for distinguishing between the two periods was the Southern Longfin Goby, which ranked first by abundance and contributed 36.5% to the total number of fishes caught in 1994, compared with ranking only sixth and contributing only 8.1% in 2008-10. Furthermore, the densities of the other two abundant species of goby, the Bluespot Goby Pseudogobius olorum and particularly the Southwestern Goby Afurcagobius suppositus, were also greater in the earlier period. The fact that these species live in and/or on the substrate suggests that the benthic environment may have deteriorated during recent years. Alternatively, the conditions in 1993 and 1994 may have been particularly favourable for the successful spawning and recruitment of goby species. Among the commercial and recreational species, the overall densities of the King George Whiting Sillaginodes punctata, the Yellow-fin Whiting Sillago schomburgkii and the Sea Mullet 5 Mugil cephalus did not differ significantly between periods, whereas that of the Yelloweye Mullet was greater in 2008-10 than in 1994. The Western Australian Salmon Arripis truttaceus was moderately abundant in 2008-10 but was not caught in the earlier period. A total of 500 Blue Swimmer Crabs, Portunus pelagicus, was caught in the Leschenault Estuary using 21.5 and 41.5 m long seine nets over two years and a further 194 were obtained using pots in the last of those years. The carapace width of only 59 of those crabs (9.4%) was greater than the minimum legal carapace width for retention (127 mm) of this species. All but five of the crabs with a carapace width greater than 127 mm were males, reflecting the tendency for the males of this marine species to remain longer in the estuary. The Blue Swimmer Crabs exhibited considerable variation in the times and strengths of recruitment. Thus, for example, a large number of crabs entered the estuary in the autumn of 2010 but not in that season in 2009. Furthermore, the catches taken from seine nets and crab pots both showed that recruitment of crabs into the estuary was greater in 1996/97 and 2009/10 than in 1997/98. Such interannual variations in estuarine recruitment are just as likely to be due to the influence of conditions in marine waters, where the eggs develop into zoeae and then juveniles, as they are to conditions in the estuary itself. Main findings and conclusions 1) The following inferences regarding similarities and differences in the fish faunas of the Leschenault Estuary in two different periods are based on comparisons between data for two years in the more recent period (2008/09, 2009/10) and those for only a single year in the earlier period (1994). As most of the dominant species found in estuaries have short life cycles and the extent of their recruitment can thus vary markedly among years and thereby lead to differences 6 in species composition, even between successive years, caution should be exercised when drawing inferences